The present invention relates to a spark igniter of the type used in turbine engines, including aircraft jet engines. Such igniters are frequently surface gap spark plugs in which a high energy spark discharge occurs between a center electrode and a ground electrode, traveling along the surface of a ceramic member. The spark discharge in such igniters is of the "high energy type" because of the nature of the ignition system used to cause sparking. The system includes a condenser which is charged as the voltage applied thereto and across the igniter increases; when the applied voltage becomes sufficiently large to cause a spark discharge the electrical energy stored by the capacitor is discharged, flowing across the spark gap. The stored energy in capacitor discharge ignition systems that are used with jet aircraft engines is usually at least one joule.
Electrode erosion has been a problem with spark igniters used with turbine engines for jet aircraft, sometimes constituting the limiting condition with respect to igniter life. Problem erosion of both the center electrode and the ground electrode occurs in igniters used with turbine engines. Conventional igniter ground electrodes are frequently made from inconel or from other conventional nickel alloys because, although they erode at a relatively rapid rate under service conditions, they are relatively inexpensive. A solution to the problem of electrode erosion in such igniters is suggested in U.S. Pat. No. 3,691,419, Van Uum et al.; this patent discloses an igniter of the type in question having a center electrode with a firing end made of spark resistant metal such as tungsten and a ground electrode having a ductile iridium ring welded therein and positioned so that it is immediately adjacent the spark gap. In the igniter of the Van Uum et al. patent, the ground electrode to which the iridium ring is welded is a portion of the metal shell of the igniter, a common structure.
It has been found that iridium and other precious metal rings, if they can be obtained at all, are extremely expensive. On the basis of price quotations that have been received, it has been estimated that the use of an iridium ring of the the type suggested by the Van Uum et al. patent in an igniter that is presently commercially available would approximately double the cost of that igniter. It has also been found that the differences in thermal expansion characteristics between iridium and the nickel alloys commonly used as ground electrode materials therein can cause catastrophic failure of igniters of the type suggested by Van Uum et al.
Various suggestions* have also been made for reducing electrode erosion in conventional spark plugs where the spark discharge occurs through a gas-filled gap between center and ground electrodes. What it calls a spark plug with "a multiplicity of semi-surface spark gaps" is suggested in U.S. Pat. No. 2,591,718 to Paul; this patent discloses a structure wherein a center electrode terminates flush with an insulator end and is in spark gap relation along the insulator end with four rod-type electrodes each of which just touches the insulator surface. FNT *See, for example, U.S. Pat. Nos. 2,391,455; 2,391,456; 2,391,458; 2,470,033 (all to Hensel); and 2,344,597 (to Chaston et al.). The Chaston et al. patent discloses a ground electrode made of a molybdenum platinum alloy wire which constitutes an insert in the metal shell of a conventional spark plug.